In general, the so-called “electrochromism” is a phenomenon in which a color change arises depending on potentials of an applied electric field. Use of the electrochromism results in production of electro-photoswitchable devices such as electrochromic devices, information memory devices, and solar cells. Typical electrochromic materials include inorganic metal oxides such as tungsten trioxide (WO3), nickel oxide (NiO) and titanium dioxide (TiO2), and organic electrochromic materials including bipyridinium salt (viologen) derivatives, quinone derivatives such as anthraquinone and azine derivatives such as phenothiazine.
Though the electrochromism was known in 1961, practical use and commercial mass production of electrochromic devices have been limited because of their shortcomings, such shortcomings being that it is difficult to realize multiple colors, coloring/bleaching rates are low, it is difficult to accomplish complete bleaching, and electrochromic materials tend to be damaged easily during repeated coloring/bleaching cycles due to their poor stability.
U.S. Pat. No. 5,441,827 (Graetzel et al.) discloses a device such as a photocell, photochromic device or an electrochromic device, having high surface area of an electrode, high concentration of electroactive materials, high efficiency and fast response speed, the device being manufactured by coating an electroactive organic material, as a monolayer, onto the surface of a nanoporous metal oxide thin film electrode obtained by sintering metal oxide nanoparticles. The device substantially solves the problems with which electrochromic devices according to the prior art are faced.
PCT International Publication No. WO 98/35267 (Fitzmaurice et al.) discloses an electrochromic device capable of repeating coloring/bleaching cycles 10,000 times or more at room temperature, the device being a more specified version of the above-mentioned metal oxide thin film-based electrochromic device. However, lifetime of electrochromic devices should be increased to 100,000 cycles or more in order to commercialize electrochromic devices and to expand application of electrochromic devices.